A widely practiced technique for the manufacture of soft contact lenses involves the latching procedure. This technique has many drawbacks inasmuch as it is a labor intensive operation, requires several steps, is relatively expensive, and the finished lens product is characterized by striations on its optical surfaces. In the lathing technique, an appropriate polymerization medium is first polymerized into a cylindrical shape from which there are cut so-called lens "buttons" or lens blanks, or the lens blanks per se can be made in appropriate molds. The blanks are subjected to a postcure treatment to improve certain of their physical characteristics. A predetermined curved surface is thereafter cut on one face of the blank by using precision lathe machinery and the cut curved surface is polished to an optical surface. Formation of an optical surface on the opposite face of the blank requires adhering the partially cut blank to an arbor or mandrel by means of a waxy substance in a manner that the uncut face of the blank is exposed for the lathing and polishing operations. Thereafter, there are washing and cleaning steps to remove residues from the cutting and polishing procedures and eventually, as with soft contact lenses, soaking in a physiologic solution until osmotic equilibrium is reached at which stage the hydrogel lens attains its final dimensions. This method suffers from the disadvantage of requiring highly skilled artisans for quality production.
In utilizing closed mold system for casting contact lenses, the inevitable shrinkage that occurs on polymerization can result in surface and/or edge irregularity of the cast lenses. With some monomeric materials, the volumetric shrinkage during polymerization can be as high as 22%. This degree of shrinkage will generally prevent satisfactory casting of soft contact lenses where surface finish and sharpe are important. For examle, as taught in U.S. Pat. No. 3,660,545, a polymerizable acrylate, or methacrylate ester mixture held in a closed glass mold invariably would pull away from at least one mold surface and cause the formation of surface voids which render the cast lines unsuitable for use as a contact lens for humans.
A closed mold casting system in which the disadvantage associated with shrinkage of the polymerizable or curable material has been proposed in U.S. Pat. No. 4,121,896 to T. H. Shepherd. The disclosure made in this application, to the extent intended herein, is incorporated by reference as if set out in full text. According to the process described in this U.S. patent, complementing replica male and female mold members each having an optical molding surface are prepared by injection molding of plastic composition in the master mold pairs. The plastic mold members are then longitudinally aligned and used to mold contact lenses, one of the molding surfaces being provided with a flexible circumferential rim which defines the periphery of the resulting lens. The lens-forming polymerizable or curable material which will constitute the lens is charged into the female mold member or portion. The complementing male member or portion is brought into contact with the female mold member so that the flexible rim portion is seated on the opposite molding surface. During molding the lens-forming material contracts causing the flexible rim to flex, inwardly usually, whereby the two mold member move towards each other. Once the molding cycle is completed the mold members are separated and the resulting contact lens is secured within the circumferential rim on the mold. The lens can then be removed in a condition ready for use, except for cleaning and perhaps a buffing of the lens edge.
Although the above-described closed mold casting process does produce precision lenses, time-consuming steps must be taken to oftentimes remove undesirable flash on the peripheral edge of the lenses as by buffing in order to make them commercially acceptable. If a male mold containing the lens is positioned on a revolving mandrel and a conventional cutting tool is employed to trim a selected peripheral edge portion of the lens, then due to the possibility of eccentricity between the opening in the cylindrical mold and the outer diameter of the mold, an asymmetrical trim could be imparted to the lens. This could result in unacceptably soft contact lenses being produced.